1. Field of the Invention
The present invention relates generally to a driving apparatus for driving lenses and other parts using a transducer such as an electro-mechanical transducer. More particularly, the invention relates to a driving apparatus which drives such optical system lenses as the photographic lenses of cameras, projection lenses of overhead projectors, binocular lenses and copier lenses, the apparatus being also used in driver-equipped devices such as plotters and X-Y drive tables.
2. Description of the Related Art
There have been proposed impact type driving apparatuses using an electro-mechanical transducer having a piezo-electric element for driving component parts in cameras and other precision equipment. One such driving apparatus is disclosed in Japanese Patent Laid-Open No. Hei 4-69070. The disclosed apparatus will now be outlined with reference to FIGS. 50 and 51. The mechanical parts shown in FIG. 50 may also be utilized in implementing the driving apparatus of this invention.
FIG. 50 illustrates a lens driving apparatus in which reference numeral 201 is a lens supporting barrel and 203 is a guide bar that supports and guides the barrel 201 in the optical axis direction. The guide bar 203 supporting and guiding the barrel 201 penetrates a fork 201f formed on a support member 201e extending from the barrel 201.
Reference numeral 217 is a driving member that doubles as a barrel support member. The member 217 supports the barrel 201 in cooperation with the support member 201e while driving the barrel 201 axially. The driving member 217 penetrates bearing holes 213b and 213d formed respectively on bearings 213a and 213c. The bearings 213a and 213c are furnished on a support member 213. Located as shown, the driving member 217 is movable axially. The driving member 217 further penetrates holes 201b and 201d formed respectively on two ends 201a and 201c of a bracket 201k. The bracket 201k extends from the barrel 201 in the opposite direction of the support member 201e. The rear end of the driving member 217 is fastened to the front end of a piezo-electric element 212. The rear end of the piezo-electric element 212 is fixed to another end plate 213e of the support member 213.
In FIG. 50, a plate spring 214 is attached with screws 215 and 216 to the two ends 201a and 201c of a bracket 201k from below. The plate spring 214 is positioned so as to be parallel to the driving member 217. In the approximate middle of the plate spring 214 is a friction part 214c protruding upward. When coming into contact with the driving member 217, the friction part 214c produces a frictional force between the barrel 201 (whose contacting part is the bracket 201k) and the driving member 217. The frictional force drives the barrel 201 that is frictionally coupled. The frictional force derives from and varies with the spring pressure of the plate spring 214.
A driving circuit 205 supplies the piezo-electric element 212 with a voltage. The piezo-electric element 212 is expanded or contracted in accordance with the supplied voltage. A position detecting circuit 208 detects the position of the barrel 201. When the barrel 201 is driven and detected to have reached its target position, the detecting circuit 208 orders the driving circuit 205 to stop. This terminates the supply of the voltage to the piezo-electric element 212.
As the piezo-electric element is expanded or contracted by the supplied voltage in order to actuate the driving member 217 in its longitudinal direction, the resulting frictional force moves the frictionally coupled barrel 201 accordingly. When the barrel 201 reaches its target position, the expansion or contraction of the piezo-electric element 212 ends and the driving action comes to an end.
Suppose that with the driving member 217 positioned horizontally, N stands for the vertical drag of the driving member 217 against the pressure from the plate spring 214, .mu. for the coefficient of static friction between the driving member 217 and the barrel 201, and Mm for the moving member mass involved. In that case, the maximum coefficient of static friction Fs between the driving member 217 ad the barrel 201 is given as: EQU Fs=.mu.(Mm.multidot.g+N)
If the coefficient of dynamic friction is assumed to be represented by .mu.', the dynamic frictional force Fd of this example is given as: EQU Fd=.mu.'(Mm.multidot.g+N)
As the driving member 217 is moved, the resulting frictional force moves the barrel 201 accordingly. If the acceleration of the driving member 217 is sufficiently small, the barrel 201 can move without sliding over the driving member 217. Once beyond a critical value, the acceleration causes the barrel 201 to slide over the driving member 217. The critical value Al for the acceleration is given as: EQU Al=Fs/Mm
With conventional driving apparatuses of the above kind, when the driving member is moved in the target direction, the acceleration of the driving member (i.e., barrel) is set below the critical value Al so as not to let the moving member slide over the driving member; when the driving member is moved opposite to the target direction, the acceleration of the driving member is set beyond the critical value Al so as to have the moving member slide over the driving member.
Referring to FIG. 50, suppose that the barrel 201 (i.e., moving member) is driven in the direction of the arrow a (in the direction in which the lens is moved) by the piezo-electric element 212. In this example, the voltage applied to the piezo-electric element 212, displacements, velocities and acceleration are shown in FIG. 51 through FIG. 54. In FIG. 52 through FIG. 54, solid lines (a) represent the characteristics of the driving member 217 and broken lines (b) denote those of the moving member (barrel) 201.
FIG. 51 shows a typical waveform of pulses applied to the piezo-electric element 212. At each pulse, the voltage is slowly raised and then rapidly turned off. This keeps the acceleration from exceeding the critical value while the voltage is being applied to the piezo-electric element 212. With the acceleration held below the critical value, the moving member moves together with the driving member. When the voltage is turned off, the acceleration exceeds the critical value Al so that the moving member remains substantially stationary (or slightly retracted under some circumstances) whereas the driving member alone returns to its initial position. Repeating this action translates the reciprocal motion of the driving member into a unidirectional movement of the moving member. FIG. 52 depicts how the conventional driving and moving members are displaced over time.
FIG. 53 indicates typical velocities at which the driving member and the moving member are moved, and FIG. 54 illustrates how the driving member and the moving member are typically accelerated over time. As shown, the higher the frequency of the supplied voltage (i.e., driving frequency), the higher the velocity of the moving member.
The driving member and the moving member (i.e., barrel 201) may also be coupled frictionally as shown in FIG. 55. In FIG. 55, a lens barrel 221 (moving member) is furnished peripherally with notched sleeves 222 and a groove 223. A driving shaft 225 (driving member) penetrates the sleeves 222, and a support shaft 226 is engaged with the groove 223. Springs 227 engaged with the grooves of the sleeves 222 are pressed against the driving shaft 225. This generates an appropriate frictional force between the contacting parts.
In acquiring their driving force, the above-described driving apparatuses using the electro-mechanical transducer utilize the frictional coupling force between the moving member and the driving member attached to the transducer. The driving force thus obtained is not sufficient where the frictional coupling force is not enough between the driving and the moving member, e.g., in a setup where lubricant is introduced between the driving and the moving member, or in a case where the contact surface of the moving member needs to be specularly finished with regard to the other parts involved.
When driving apparatuses of this kind are used in cameras or other portable devices, the positional relationship between the driving and the moving member varies with the attitude in which the portable device is operated. This causes the frictional coupling force between the driving and the moving member to fluctuate, making it difficult to ensure stable driving force.
In the examples of FIGS. 50 through 55, attempts could be made to increase the recoiling force of the plate spring so as to generate a sufficient frictional coupling force between the moving member and the driving member equipped with the plate spring. In such cases, the contacting portions form a point or a line. This can result in an abnormally increased contact pressure concentration making the acquisition of steady driving force difficult.
The above-described driving apparatuses using the electro-mechanical transducer such as the piezo-electric element utilize the expansion and contraction of that transducer for the actuating motion. The velocity of the moving member is associated with the frequency of driving pulses applied to the electro-mechanical transducer and with the amount of displacement of the transducer fed with such driving pulses. Under this condition, as long as the displacement of the electro-mechanical transducer varies more or less proportionately depending on the driving pulses applied thereto, the velocity of the moving member may be increased by raising the frequency of the driving pulses.
When the velocity ratio of expansion to contraction of the electro-mechanical transducer (i.e., expanding velocity/contracting velocity) is sufficiently high (2, 3 or higher from the inventors' experiments), the amount of movement of the moving member is close to the amount of the electro-mechanical transducer activated by each driving pulse applied to the latter.
However, electro-mechanical transducers have a physical characteristic called a delayed response. The delayed response is a phenomenon wherein the electro-mechanical transducer produces a displacement upon elapse of a certain period of time following the supply of a driving pulse. Because of this characteristic, higher frequencies of the driving pulses applied to the electro-mechanical transducer are more liable to cause the expanding and contracting displacements to overlap. The phenomenon eventually reduces the velocity ratio of expansion to contraction of the electro-mechanical transducer. Furthermore, the delayed response time varies with the mass of the driving and moving members as well as with the elastic modulus thereof.
When the electro-mechanical transducer is supplied with sawtooth driving pulses such as those used in the examples above, the transducer generates vibration called ringing. Thus the electro-mechanical transducer adds the ringing vibration to the displacement it generates. When the displacement of the electro-mechanical transducer is supplemented by the ringing vibration, the driving member coupled to with the transducer vibrates simultaneously. This reduces the velocity of the moving member in motion.
It is a known fact that the ringing vibration is avoided by making the rise time of each driving pulse an integer multiple of the resonance frequency of the driving system in use. However, the moving member is moved only when the rise time of each driving pulse is made shorter than the fall time thereof. This means that the rise time of the driving pulse must be less than half of, or in practice much smaller than, each driving period. Where the driving period is substantially short, the method of making the pulse rise time an integer multiple of the resonance frequency cannot be practiced.
In the conventional setup of FIG. 50, a rapid expansion or contraction of the piezo-electric element 212 causes the dynamic frictional force between the barrel 201 and the driving member 217 to shift the barrel 201 more or less together with the driving member 217. This can move the barrel 201 opposite to the desired direction, causing losses in its displacement. The greater the frictional force, the larger the driving losses. However, contrary to what might be expected from this, reducing the frictional force may not be preferable in practice for the following reasons:
The greater the inertia of the driving system, the greater the frictional force the system needs for its actuating motion. If the mass of the member to be driven becomes larger, the frictional force must be increased correspondingly to counter it. It may well be that the practiced application in question will not work unless a substantially large frictional force is somehow generated.
Furthermore, since lower frequencies of driving pulses fed to the piezo-electric element 212 can raise the vibration noise to an annoyingly audible level, relatively high driving frequencies are preferred in practice. This, however, means that even slow expansion and contraction of the piezo-electric element 212 can result in a fairly high velocity of displacement. For the barrel 201 (moving member) to by driven by the driving member 217 then requires a substantially large frictional force.
Suppose that there is provided a setup where a plate spring is pressed directly against the driving member to generate the necessary frictional force in the contacting portions. In this setup, the driving member is assumed to move reciprocally at varying velocities in the forward and reverse directions with respect to the axial direction. In such a case, the plate spring deforms elastically in its moving direction. Specifically, that part of the plate spring which contacts the driving member is deformed relative to the moving member. This means that the displacement caused by the reciprocal motion of the driving member cannot be transmitted precisely to the moving member. As a result, the moving member such as the lens barrel cannot be moved as desired.
With the above-mentioned impact type driving apparatus using the electro-mechanical transducer, the transducer fed with driving pulses of a given frequency mechanically vibrates the driving system composed of the driving member and of the moving member frictionally coupled with the driving member. This inevitably causes the driving system to generate a vibration noise corresponding to the frequency of the supplied driving pulses.
The impact type driving apparatus generally adopts driving pulses having frequencies ranging approximately from 100 to 500 Hz. When such a driving apparatus is activated, the vibrating noise is audible to the human ear. Conventionally, few measures have been taken specifically to inhibit such noise. The relatively low level of the generated noise did not encourage the effort to suppress it.
When a driving apparatus of the above kind is incorporated in portable equipment used under quiet circumstances, the vibrating noise from the driving apparatus in motion becomes conspicuous. In particular, if the equipment incorporating the driving apparatus is a camera operated illustratively in a concert hall, the noise from the activated apparatus cannot be ignored.
In the conventional setup of FIG. 50, parts of the driving apparatus cannot be replaced unless the entire setup is taken apart. This is because the driving member 217 is coupled directly to the barrel 201. Where the electro-mechanical transducer such as a piezo-electric element is used for the driving action, the reciprocating motion takes place rapidly. This causes the electro-mechanical transducer, the driving member and the moving member to suffer from impact-related breakage and from the impact-induced peeling of bonded parts more often in the above apparatus than in other kinds of driving apparatuses. Such irregularities, if developed in the above type of driving apparatus, are difficult to deal with.
The entire setup of the driving apparatus also needs to be disassembled conventionally when it is desired to repair and/or clean the driven parts (the barrel and lens in the above example).
Also in the setup of FIG. 50, it is impossible to remove the driving apparatus alone without disassembling the whole setup. This means that the driving apparatus alone cannot be tested independently for performance; it must be assembled into the target equipment, tested for performance, and dismounted again if necessary from the target equipment for further adjustment. The procedure adds redundant steps to the process of manufacturing the target product and thus leads to cost increases.
Furthermore, the conditions for efficiently activating driving apparatuses using the above-mentioned impact type actuator have yet to be fully clarified.